As Labour embarks on an intense debate on the future of Trident, a former aide to Gordon Brown has joined forces with a frontbencher to declare that the party would look "dangerously weak" if it diluted Britain's nuclear deterrent.

John Woodcock, who served as the former prime minister's spokesman, and Angela Smith, the shadow deputy leader of the Commons, issued their warning as senior figures in the party urge Ed Miliband to use a government review of Trident to support a smaller deterrent.

But in a Guardian article Woodcock and Smith warn: "While the world has changed greatly since the 1980s, the political reality has not: we will appear dangerously weak as a future party of government if we are prepared to give up that insurance while the world remains so unstable."

The pro-Trident MPs express the hope that Miliband will resist pressure for Labour to change tack. Labour has been a wholehearted supporter of Trident for the best part of 20 years after it resolved a bitter debate in the 1960s, 70s and 80s between advocates of unilateral and multilateral disarmament.

The two write: "The unilateralists and those pushing the mini-deterrent fantasy will be disappointed. Labour under Ed's leadership would never hand a gift to opponents by opting for a plan that might look fine in a Liberal Democrat election leaflet but would create a credibility gap in the eyes of the electorate, and do major damage to Britain's manufacturing base."

The intervention by Woodcock and Smith comes on the day CND holds a rally against Trident at the Aldermaston Atomic Weapons Establishment and after members of Labour's anti-Trident wing recently briefed the FT that the leadership is open to a downgrading of the deterrent.

The wing is hoping to seize on the government review led by the Lib Dem Treasury chief secretary, Danny Alexander, which is examining alternatives to Trident.

The Lib Dems have raised the possibility of a land or air-based system or carrying smaller nuclear warheads on cruise missiles aboard Astute class submarines. The Conservatives are committed to replacing Trident with a "continuous at-sea deterrent" (CASD) in which, similar to current practice, nuclear warheads on intercontinental ballistic missiles are carried on four Vanguard-class submarines.

The key decision on a Trident replacement, known as the "main gate", has to be taken in 2016. This is when Britain will have to decides whether to spend between £25bn and £30bn on replacing the four Vanguard-class submarines, which are due to be taken out of service in the mid 2020s, with a Successor class. The Trident missiles are due to remain in service until at least 2042.

The timing of thate "main gate" decision means Trident could be a major issue in the 2015 general election. Labour opponents of Trident are hoping to find common cause with the Lib Dems.

Woodcock is MP for Barrow-in-Furness, where the Successor submarines would be built. Many of Smith's Penistone and Stocksbridge constituents work at Sheffield Forgemasters, one of the companies that would make components for the new submarines.

The Labour leadership, which is wholly committed to maintaining Britain's nuclear deterrent, is keen to take the Lib Dem review seriously. It could opt for a mild change such as agreeing that the Successor submarines would number just three, an idea suggested at one point by Gordon Brown.

The leadership may support reducing the number of warheads on board the submarines following reductions in 1999 and 2010. This would allow Britain to make a contribution to disarmament.

But it is wary of some of the most radical proposals being examined by the Lib Dems such as placing nuclear warheads on cruise missiles on the smaller Astute submarines.